1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, to a cap assembly for a secondary battery.
2. Description of the Related Art
A secondary battery may be recharged for repeated use. Secondary batteries may be made to small sizes and large capacities, and are cylindrical, prismatic, button-shaped, etc. depending on an external shape of a case, which holds an electrode assembly (i.e., pole plate assembly).
The prismatic battery will be used as an example to describe a structure of the conventional secondary battery. The basic structure of the conventional prismatic secondary battery includes a prismatic can; an electrode assembly formed in a jelly roll configuration by interposing separators between anode plates and cathode plates, on which an active material is deposited, the electrode assembly being provided in the can; and a cap assembly provided to an opening of the can to seal the same, the cap assembly being electrically connected to the electrode assembly.
The cap assembly includes a cap plate integrally assembled to the can to form a seal with the same; a tab welding plate for fixing an electrode tab that is connected to one of the electrode plates of the electrode assembly (e.g., one of the cathode plates); an insulating member provided as insulation between the cap plate and the tab welding plate; an electrode pin that passes through holes formed in the cap plate, the tab welding plate, and the insulating member to contact the tab welding plate for electrical connection to the same; and a gasket providing insulation between the electrode pin and the cap plate.
However, with the use of the cap plate having the structure described above by the secondary battery, in the case where the gasket experiences problems such that it undergoes deformation, electrolyte within the can easily leaks such that battery performance is reduced. This is a result of an area between the electrode pin and the gasket that is closely contacted, that is, a connecting area of a head portion and a column of the electrode pin being smooth and flat such that if the gasket is deformed, the electrolyte within the can travels along the column of the electrode pin and easily exits the can through the head portion.
Further, a bottom surface of the insulating member that sits on the tab welding plate is flat such that during assembly of the cap assembly, when calking of the electrode pin to the cap plate, the insulating member, and the tab welding plate is performed, the force used during calking is applied to all of the insulating member such that a significant reactive force is generated from the insulating member. This reactive force acts to deform the insulating member itself such that the seal formed at the bottom of the cap assembly is broken or made less effective.
Finally, in the secondary battery having the cap assembly as described above, it is necessary to provide a separate space for the placement of the electrode tab between the tab welding plate and the electrode assembly. This interferes with attempts to maximize the volume of the electrode assembly within the can, which, in turn, makes increasing battery capacity difficult.